Method of treating uncured thermosetting resin matrices

ABSTRACT

Treating an uncured resin to impart energy to the resin enables a stable Tg to be achieved whereby the resin may be stored and has a low tack enabling subsequent processing and handling. The invention provides a method of achieving a stable Tg without the resin starting to cure and a method of determining the treatment regime by which a resin with a stable Tg may be obtained. The resin may be fresh or reused uncured resin and may contain fibrous reinforcement.

This invention relates to a method of producing a stabilised uncured thermosetting resin matrix and to a method of determining a treatment regime for stabilising the glass transition temperature (Tg) of an uncured thermosetting resin matrix and to a stabilised uncured thermosetting resin matrix.

Curable thermosetting resin matrices or resin matrix compositions are employed in a range of applications including in composites which comprise fibrous reinforcement materials impregnated with the cured resin matrix and in so-called “prepregs” which comprise fibrous reinforcement materials impregnated with the uncured resin matrix which may then be processed to form a reinforced composite material. Such composite materials are typically lightweight and of high strength and are used in many structural applications such as in the automobile and aerospace industries and in industrial applications such as wind turbine components such as spars and the shells used to make the blades. Such applications typically require the prepreg and composite to comply with stringent requirements, often stipulated by the manufacturer of products for such applications, as regards handling, processing and storage of the materials especially where safety considerations are paramount. The thermosetting resin matrix composition (in short “matrix”) comprises at least one resin polymer component and at least one curative. The curative enables the resin component to form an interpolymer network upon curing. Curing is achieved by imparting energy to the resin matrix composition, preferably the energy is in the form of heat.

Prepregs may be produced by a range of methods which typically involve impregnation of a moving fibrous web with a liquid, molten or semi-solid uncured thermosetting resin matrix composition. The thermosetting resin matrix may be cast on a substrate before it is applied to the reinforcement material or alternatively, the thermosetting resin matrix composition may be applied directly to the fibrous reinforcement material (direct impregnation). Prepregs may also be manufactured by exposing the fibrous reinforcement to a solvated thermosetting resin matrix composition which is then followed by flashing off of the solvent.

Prepreg is typically shipped to end-users on a roll. The end-user invariably cuts the prepreg to a desired shape and lays this up in relation to a mould to form curable stacks. These are then cured to form composite parts.

The lay-up of prepreg inevitably results in sections of unused prepreg (off cuts). Also, the production process of the prepreg may result in unused sections or volume of resin matrix.

Also, the production and the use of prepreg result in a list of excess material types such as selvedge, end of rolls and off cuts all being at uncured state. As generated unintentionally through a production process, those materials can be identified as by-products. In this application we will refer to the unused resin matrix composition (or resin matrix) and the unused prepreg as “by-product”.

Due to safety considerations and stringent specification relating to handling, processing and storage of thermosetting resins matrices and prepregs, re-use of by-products has been problematic. Furthermore, this activity has not proved economically viable to date. Whilst recycle and re-use of dry carbon fibre waste is known on a commercial scale, the costs and technical difficulties of “recycling” prepregs by pyrolysis (thermal degradation) and solvolysis (chemical dissolution) of the resin matrix has presented difficulties due to high costs and energy requirements to process the resin and in the case of prepregs, to remove the resin from the fibrous reinforcement together with waste product not necessarily being suitable for re-use in the supply chain for products which are subject to stringent performance and safety specifications.

US2015/0151454 describes a method and system for recycling uncured composite offcuts comprising reinforcing fibres and uncured polymer matrix material by mixing the uncured composite material in a mixing device to blend the fibres and matrix material into a generally homogeneous mixture and feeding the mixture from the mixing device to form a component or a semi-finished product. The offcuts are fed into the mixing device directly without pre-treatment of the offcuts.

Unused resin and prepregs may be stored in a wide range of ambient conditions from sub-zero to high temperature and humidity depending on the prevailing climate in the storage location and may undergo changes in properties, particularly in Tg. Where such materials are processed into a prepreg or composite product and waste resin material is generated, re-use of such material may not be possible in applications where it is necessary to be able to guarantee the conditions under which the resin has been stored prior to use. A need remains for uncured resins to be suitable for use in the forming prepregs and composite products irrespective of the prior storage conditions under which the properties of the resin may have varied.

The invention aims to obviate or at least mitigate the above described problems and/or to provide improvements generally.

According to the invention there is provided a method or process and a composition as defined in any one of the accompanying claims.

We have now found that by treating an uncured resin to impart energy to the resin matrix without advancing the cure of resin matrix composition to a value above 60% of its total cure enthalpy, preferably above 50% of its total cure enthalpy and more preferably above 40% of its total cure enthalpy, or more preferably above 30% of its total cure enthalpy or more preferably above 20% or above 10% of its total cure enthalpy, the Tg of the resin may be stabilised such that it does not change significantly over an extended period. The uncured resin may then be suitable for processing to form a prepreg or composite having the properties required for a particular application.

The glass transition temperature or Tg is determined in accordance with ASTM D3418 using Digital Scanning Calorimetry (DSC).

The unused resin matrix and/or unused prepreg may have a residual cure of greater than 5% of their total cure enthalpy, or greater than 10% of their total cure enthalpy, or greater than 15% of their total cure enthalpy, or greater than 20% of their total cure enthalpy, or greater than 30% of their total cure enthalpy, or greater than 40% of their total cure enthalpy. In this context, “unused” or “uncured” as used interchangeably in this application is thus defined by the residual cure available in the unused resin matrix composition or unused prepreg.

Digital Scanning Calorimetry is utilized to determine % cure and reaction enthalpy. The total heat or reaction enthalpy detected during the DSC measurement is identified as the heat released by the curing reaction when the resin matrix composition is heated from a starting temperature of typically 10° C. (or room temperature of 21° C.) to a temperature at which cure is anticipated to be completed. For fast cure epoxy resins the temperature at which cure is anticipated to be fully completed is typically 100 to 225° C., preferably from 100 to 160° C. and the ramp rate for the temperature is typically set at 10° C./s or faster rate.

Once the total heat enthalpy has been established, the residual cure of any subsequent test sample of the resin which has been subjected to a particular cure can then be analysed by exposing the test sample to the same heat up rate and the remaining reaction enthalpy is determined using DSC. The degree of cure of the test sample is then given by the following formula: cure %=(Δ Hi−Δ He)/Δ Hi×100 where ΔHi is the heat generated by the uncured resin heated from the starting temperature up to the anticipated fully cured temperature (in the present invention typically 150° C.) and ΔHe the heat generated by the test sample heated up after initial cure to it being fully cured at 150° C. (so ΔHe represents the residual enthalpy which is released following complete curing of the sample following on from the initial cure schedule).

The invention provides in a first aspect a method of producing a stabilised unused resin matrix composition comprising providing a resin matrix composition having an initial Tg and subjecting the resin to:

-   -   I. a first regime comprising imparting energy to the resin         matrix composition to raise the Tg by at least 5° C. to provide         a raised Tg without substantially curing the resin; and     -   II. a second regime comprising storing the resin matrix         composition to provide a stabilised resin matrix composition         wherein the Tg of the stabilised uncured resin is such that it         does not increase by more than 10° C. from the raised Tg when         stored for at least 24 hours at room temperature (21° C.).

In Step I. “without substantially curing” means that the residual cure of the resin matrix is reduced by a maximum of 20%, preferably a maximum of 10% and more preferably a maximum of 5%. In Step I the imparting of energy without substantially curing of the matrix composition results in a reduction of residual cure enthalpy in the range of from 1 to 20%, preferably from 2 to 15% and more preferably from 5% to 10% and/or combinations of the aforesaid ranges.

The curing reaction of the resin matrix composition progresses more rapidly during the step of imparting of energy but this is arrested or reduced to a very low rate in Step II when the resin matrix composition is stored at a temperature below room temperature or at room temperature (21° C.) respectively.

The duration of step I is selected so minimize the impact on the residual cure enthalpy of the resin matrix composition. The duration is in the range of from 72 hours to 30 mins, preferably from 48 hours to 1 hour, more preferably from 24 hours to 4 hours, more preferably from 18 hours to 6 hours and/or combinations of the aforesaid ranges.

Step I of the process of the invention is sometimes referred to as “staging”.

To determine whether curing commences and so to determine the conditions to be applied in treating the resin, methods known in the art may be employed, for example FTIR. In this respect we refer to “Applications of FTIR on Epoxy Resins—Identification, Monitoring the Curing Process, Phase Separation and Water Uptake”, M González González, J C Cabanelas, J Baselga, as published in “Infrared Spectroscopy—Materials Science, Engineering and Technology”, Edited by Prof. Theophanides Theophile, 2012 In a preferred embodiment, energy is imparted to the resin matrix composition by heating.

This may occur in a number of ways: by heat transfer, radiation heating, ultrasound, microwave radiation, or infrared light.

Suitably, upon imparting energy to the resin matrix and following step I of the process of the invention, the raised Tg of the resin is at least 5° C., preferably at least 20° C., desirably at least 30° C. and especially greater than 50° C. The raised T_(g) is preferably not increased beyond a value greater than preferably at least 40° C., desirably at least 60° C. and especially greater than 80° C. in comparison to its original T_(g) before the start of the process of the invention.

The resin suitably has a Tg above the ambient temperature under which it is likely to be used or stored thereby reducing the likelihood of the uncured resin being tacky and being susceptible to agglomeration which may present processing difficulties. Most fundamentally, given that Tg is linked to polymer backbone mobility, a Tg higher than ambient decreases the likelihood that the resin matrix will exhibit enough mobility to undergo continuing cross-linking reactions. The resin having a stabilised Tg may advantageously be handled and cut without problems occurring due to its tackiness or its flow (such as gumming and adherence)

A resin matrix composition treated according to the invention which has a Tg in the range of from 5° C. to 20° C., desirably from 10 to 30° C., more preferably from 15° C. to 50° C., and even more preferably from 20° C. to 80° C. and/or combinations of the aforesaid ranges. The invention provides storage over an extended period at ambient temperature without there being significant clustering or agglomeration of the resin when in the form of discrete elements irrespective of whether the storage temperature is regulated. Advantageously, this allows resins to be stored in a range of ambient conditions without the need to provide refrigeration or cooling areas to ensure the stored resin remains usable in downstream processes.

Energy may be imparted to the uncured resin in the first regime by any suitable means, for example heating, pressure and microwave, holding the resin matrix composition at room temperature. Preferably, energy is imparted to the uncured resin matrix composition by heating to an elevated temperature for a first period of time. The resin may be subjected to pressure to impart energy to the resin. Suitably, the resin matrix composition is heated to an elevated temperature from 30° C. up to a temperature below the temperature at which the resin commences curing, preferably from 30 to 70° C. Suitably, energy is imparted to the resin matrix composition in the first regime for a period of 1 to 100 hours, preferably the first regime has a duration in the range of from duration is in the range of from 72 hours to 30 mins, preferably from 48 hours to 1 hour, more preferably from 24 hours to 4 hours, more preferably from 18 hours to 6 hours and/or combinations of the aforesaid ranges.

Once energy has been imparted to the resin matrix composition in the first regime, the resin matrix will have a raised Tg and is then suitably subjected to a second regime under which the raised Tg may increase. The uncured resin is subjected to that regime until the Tg remains stable in that it does not increase by more than 10%, preferably more than 8%, or 6% or 5% or 2% or 1% from the raised Tg when stored for at least 1 day or 2 days or 4 days or 5 days or 7 days or 14 days and/or combinations of the aforesaid percentages and durations.

Preferably the temperature in the second regime is ambient. In the second regime the resin is suitably stored for at least 12 hours at ambient temperature or a temperature which is below ambient. Preferably the second regime comprises storing the resin for a period of at least 24 hours to 10 weeks at ambient temperature.

Different resin matrix compositions may require different conditions to impart sufficient energy to the resin to provide a stable Tg whilst not commencing curing. Where the resin matrix composition has been previously used in the formation of a prepreg or composite, the conditions under which the resin matrix composition has been stored or processed may also influence the particular stabilising treatment regime required in order to provide an uncured resin matrix composition having a stabilised Tg. Suitably the conditions of the first regime and the second regime in the method for providing a stabilised uncured resin matrix composition are determined by subjecting an uncured resin matrix composition of a particular type of from a particular source to a method of determining the treatment regime for stabilising the glass transition temperature (Tg) of that uncured resin matrix composition.

In a second aspect, the invention provides a method of determining a treatment regime for stabilising the glass transition temperature (Tg) of a curable resin matrix composition comprising:

-   -   i) providing an uncured or unused resin matrix composition         having an initial Tg and selecting a first regime comprising an         elevated temperature and residence time to impart energy to the         uncured or unused resin matrix composition whereby the residual         cure of the resin matrix is reduced by a maximum of 20%,         preferably a maximum of 10% and more preferably a maximum of 5%;     -   ii) imparting the selected energy over the selected residence         time so as to increase the Tg of the resin matrix composition;     -   iii) selecting a second regime comprising storing the resin         matrix composition at a selected second temperature and second         residence time wherein the second regime is different to the         first regime, the selected temperature and residence time being         selected so that the Tg does not increase by more than 10%,         preferably more than 8%, or 6% or 5% or 2% or 1% ° C. from the         raised Tg when stored for at least 1 day or 2 days or 4 days or         5 days and/or combinations of the aforesaid percentages and         durations;     -   iv) subjecting the resin matrix composition to the second         regime;     -   v) measuring the raised Tg of the resin matrix composition prior         to or at commencement of the second regime and measuring the Tg         of the resin matrix composition periodically until over a         selected period of time ranging from 12 hours to 144 hours or         periods thereof either:         -   a. the periodically measured Tg does not increase by more             than 1%, or 2%, or 4%, or 5% or 10% from the raised Tg             thereby determining the conditions of the first regime and             the second regime; or         -   b. where the periodically measured Tg increases by more than             1%, or 2%, or 4%, or 5% or 10% from the raised Tg, modifying             the first regime and/or second regime and repeating steps             ii), iv) and v) until the periodically measured Tg does not             increase by more than 1%, or 2%, or 4%, or 5% or 10% from             the raised Tg.

The temperature and duration to which the resin matrix composition is subjected in the first regime may be selected having regard to the temperature at which the resin matrix composition commences curing or proceeds through to full cure or proceeds to cure at a high rate. The particular resin matrix composition which is to be treated may be heated and analysed using known methods of determining Tg known in the art to determine this temperature. Suitably, a temperature below that level will be appropriate to provide a means of imparting energy to the resin matrix composition without curing commencing. Desirably, the resin matrix composition is heated to an elevated temperature from 30° C. up to a temperature below the temperature at which the resin matrix composition commences curing, preferably from 30 to 110° C. Suitably, energy is imparted to the uncured resin matrix composition in the first regime for a period of 1 to 144 hours, preferably in the range of from duration is in the range of from 72 hours to 30 mins, preferably from 48 hours to 1 hour, more preferably from 24 hours to 4 hours, more preferably from 18 hours to 6 hours and/or combinations of the aforesaid ranges.

Once energy has been imparted to the resin matrix composition in the first regime, the resin matrix composition will have a raised Tg. This Tg, referred to as the “raised Tg” of the resin matrix composition is suitably measured using methods known in the art, preferably differential scanning calorimetry (DSC). The resin matrix composition is then suitably subjected to a second regime under which the Tg of the uncured resin matrix composition is measured periodically to determine any change, particularly an increase in Tg. Where the Tg of the resin matrix composition does not increase by more than 10% from the initial Tg following the energy imparting step and preferably by not more than 5% from the raised Tg following the energy imparting step over a period of 1 day, the resin matrix composition may be considered as having a stable Tg. If the Tg continues to vary outside these bounds, the resin matrix composition is not considered as having a stable Tg and the method repeated using the same or a different sample of the resin matrix composition.

In this case, the conditions of the first regime should be altered by increasing the temperature and/or period under which energy is imparted to the resin matrix composition and the same sample or a different sample of the uncured resin matrix composition subjected to the first and second regime with the Tg of the resin matrix composition being periodically determined in the second regime.

With knowledge of the present method of determining a treatment regime for stabilising the glass transition temperature (Tg) of a curable resin matrix composition, the skilled person will be able to modify the conditions of the first regime to determine the conditions under which the resin matrix composition may be treated to provide a stabilised Tg. The treatment regime, as determined by this method using samples of a particular resin matrix composition from a particular source may then be employed to stabilise a resin matrix composition of that type and from the same source in a method according to the first aspect of the invention.

We have found that by employing a method of stabilising an uncured resin matrix composition according to a first aspect of the invention, a treated uncured resin matrix composition having a stable glass transition temperature (Tg) at ambient temperature may be provided. The stabilised uncured resin matrix composition has a raised Tg after being treated and the Tg of the treated uncured resin matrix composition remains within 10% of the raised Tg when stored for at least 1 day.

The resin matrix composition having a stabilised Tg may then be stored and used in a downstream process of forming a prepreg comprising a fibrous reinforcement and the uncured resin matrix composition having a stabilised Tg.

Suitably, the treated uncured resin matrix composition has a Tg which remains within 10% or 5% of the raised Tg following the energy imparting step when stored for at least 1 week, preferably for at least 1 month.

Preferably, the treated uncured resin matrix composition is in the form of discrete elements. Preferably, the discrete elements have a tack F/Fref of not more than 0.1 to 0.45 at ambient temperature (21° C.) where Fref=28.19N and F is the maximum debonding force as defined with reference to WO2013087653 A1 and DUBOIS ET AL.: ‘Experimental analysis of prepreg tack’ LAMI)UBP/IFMA 5 Mar. 2009 and as summarized below.

Tack measurement is performed using the probe tack test as disclosed in the above Dubois etal. paper. A 1 kN Instron 5543 universal testing machine (Norwood, Mass., USA) is used. The upper grip of the machine is replaced by an aluminium cylindrical probe of 10 mm diameter mounted on the moving crosshead of the machine, via a 50 N capacity load cell. The probe can be heated with a flexible heater. A PT100 temperature sensor linked to a Proportional-Integral-Derivative (PID) temperature controller regulates the temperature. Test samples are applied on the lower support. The probe, which is surrounded by a heater, is set at a given temperature, and then the probe is brought into contact with the sample, and the maximum reading of the load cell was recorded for each test. The test procedure used for the tack measurement is as follows: 1. Test strips of prepreg or resin matrix material are cut to meet the probe. Samples were then put in a climatic chamber for a given time at controlled temperature and Relative Humidity; 2. Before each run, the contact surface of the probe and the support are cleaned with acetone; 3. Contact time, contact force and debonding rate are set for the mechanical cycle; 4. The temperature of the probe was set by regulator; 5. Once the temperature of the probe reached the set point and was equilibrated, the sample was removed from the climatic chamber and was positioned on the support. It should be noted that the side of the sample protected by a release paper, i.e. the tackiest side, was applied without pressure on the lower grip. The release paper was then removed; and 6. The value given by the load cell was reset and the test started. Time, force and crosshead displacement were then measured.

The Dubois etal method allows tack to be measured objectively and repeatably by using the equipment as described above and by measuring the maximum debonding force for a probe which is brought in contact with the sample structure at an initial pressure of 30N at a constant temperature of 30° C. and which is subsequently displaced at a rate of 5 mm/min. For these probe contact parameters, the tack F/Fref for the tack material is in the range of from 0.1 to 0.6 or from 0.1 to 0.45 at room temperature (21° C.) where Fref=28.19N and F is the maximum debonding force.

By having a low tack, suitably at ambient temperature, the discrete elements may be readily processed and are less susceptible to agglomeration. This facilitates cutting and handling at room temperature (21° C.) for example when using the treated unused or uncured resin matrix composition in the formation of a prepreg and may reduce process downtime by reducing the requirement for servicing or cleaning process apparatus for example when forming a prepreg.

Alternatively, the discrete elements may be cooled to a temperature below ambient to reduce their tack prior to and/or during cutting and processing to enhance their processability. Typically the temperature could be reduced to −18° C.

Suitably, the treated uncured resin matrix composition comprises discrete elements which have a torque peel (T Peel) of less than 10N/10 mm and desirably less than 5N/10 mm at 20° C. T Peel is preferably determined by a method as set out in the below Examples.

The present invention may be employed with a wide-range of curable resin matrix compositions. One preferred family of resin matrix compositions contains curable epoxy resin components. The resin matrix composition may comprise other components for example thermoplastics or rubbers in the epoxy resin matrix composition. The epoxy resin material component or epoxy resin polymer or in short, epoxy resin may be selected from any of the commercially available diglycidylethers of Bisphenol-A either alone or in combination, typical materials in this class include GY-6010 (Huntsman Advanced Materials, Duxford, UK), Epon 828 (Resolution Performance Products, Pemis, Netherlands), and DER 331 (Dow Chemical, Midland, Mich.).

The Bisphenol-A epoxy resin component preferably constitutes from 30 to 50% w/w (weight % of the component based on the total weight of the composition containing the component) of the total resin matrix and the remainder may be a thermosetting resin component material and/or a thermoplastic material.

Preferred epoxy resins have an Epoxy Equivalent Weight (EEW) in the range from 150 to 1500 preferably a high reactivity such as an EEW in the range of from 200 to 500 and the resin composition comprises the resin and an accelerator or curing agent. Suitable epoxy resins may comprise blends of two or more epoxy resins selected from monofunctional, difunctional, trifunctional and/or tetrafunctional epoxy resins.

Suitable difunctional epoxy resins, by way of example, include those based on: diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol A (optionally brominated), phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldelyde adducts, glycidyl ethers of aliphatic diols, diglycidyl ether, diethylene glycol diglycidyl ether, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidised olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, glycidyl esters or any combination thereof.

Difunctional epoxy resins may be selected from diglycidyl ether of bisphenol F, diglycidyl ether of bisphenol A, diglycidyl dihydroxy naphthalene, or any combination thereof.

Suitable trifunctional epoxy resins, by way of example, may include those based upon phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, aromatic epoxy resins, aliphatic triglycidyl ethers, dialiphatic triglycidyl ethers, aliphatic polyglycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof. Suitable trifunctional epoxy resins are available from Huntsman Advanced Materials (Monthey, Switzerland) under the tradenames MY0500 and MY0510 (triglycidyl para-aminophenol) and MY0600 and MY0610 (triglycidyl meta-aminophenol). Triglycidyl meta-aminophenol is also available from Sumitomo Chemical Co. (Osaka, Japan) under the tradename ELM-120.

Suitable tetrafunctional epoxy resins include N,N, N′,N′-tetraglycidyl-m-xylenediamine (available commercially from Mitsubishi Gas Chemical Company under the name Tetrad-X, and as Erisys GA-240 from CVC Chemicals), and N,N,N′,N′-tetraglycidylmethylenedianiline (e.g. MY0720 and MY0721 from Huntsman Advanced Materials). Other suitable multifunctional epoxy resins include DEN438 (from Dow Chemicals, Midland, Mich.) DEN439 (from Dow Chemicals), Araldite ECN 1273 (from Huntsman Advanced Materials), and Araldite ECN 1299 (from Huntsman Advanced Materials).

The epoxy resin compositions used preferably also comprises one or more urea based curing agents and it is preferred to use from 0.5 to 10 wt % based on the weight of the epoxy resin of a curing agent, more preferably 1 to 8 wt %, more preferably 2 to 8 wt %. Preferred urea based materials are the range of materials available under the commercial name Urone®. In addition to a curing agent, a suitable accelerator such as a latent amine-based curing agent, such as dicyanopolyamide (DICY).

Another suitable product comprising resin which may be employed in the present invention is the product available from Hexcel Composites Ltd as HexMC® or HexForm® which also comprises fibres. Examples of preferred resins for use in this invention include those employed in multifunctional epoxy resin matrix compositions which are commercially available under the following trade names Hex MC® M21, HexMC®M77, HexMC® M78, HexMC® M81 and HexMC® M92 and 8552 (all as supplied by Hexcel Corporation).

Other suitable resins for use in the present invention include polyester resins and bismaleimide resins.

In a preferred embodiment, the uncured resin matrix composition is impregnated in a fibrous reinforcement material. The fibrous reinforcement material pre-impregnated with an uncured resin matrix composition may comprise a by-products of uncured composite which has previously been subjected to processing and has been discarded as an offcut in that prior process.

Exemplary fibres include glass, carbon, graphite, boron, ceramic and aramid. Preferred fibres are carbon and glass fibres. Hybrid or mixed fibre systems may also be envisaged. The use of cracked (i.e. stretch-broken) or selectively discontinuous fibres may be advantageous to facilitate lay-up of the product according to the invention and improve its capability of being shaped. Although a unidirectional fiber alignment is preferable, other forms may also be used. Typical textile forms include simple textile fabrics, knit fabrics, twill fabrics and satin weaves. It is also possible to envisage using non-woven or non-crimped fiber layers.

The invention is illustrated by the following non-limiting examples and with reference to the accompanying drawings in which

FIG. 1 presents a diagrammatic view of a lay-up of a resin matrix composition according to an embodiment of the invention using a vacuum bag;

FIG. 2 presents a diagrammatic view of gripping layers of a sample according to another embodiment of the invention; and

FIG. 3 presents a diagram showing the relationship between peel torque (in N/10 mm) in relation to Tg (in ° C.).

EXAMPLE 1

Several batches of pre-preg products comprising uncured resin matrix composition and carbon fibres were subjected to T Peel testing. The pre-preg product was as follows:

3 batches of M21E/34%/UD 194/IMA-12K:

-   -   Batch 1: N23C14 02A     -   Batch 2: N14302 12A     -   Batch 3: N14205 05A

1 batch of M21E/34%/UD 268/IMA-12K:

-   -   Batch 1*: N22A1801A

Stabilizing T_(g)

Five samples from each batch were subjected to a first regime by heating in an oven at 50° C. under a dry atmosphere for following periods: 24 h, 72 h, 144 h, 152 h and 200 h. For each batch, a sample of fresh material was kept without being subjected to the first regime. And referred to as T0 below. After the first regime, the samples were held in a freezer at −18° C. (0° F.). Furthermore, each specimen was analyzed by DSC¹ test just after the first regime and prior to the second regime to determine Tg. The Tg results are shown in Table 1. ¹Differential scanning calorimetry

The DSC cycle used for analysis consisted of a ramp of 10° C./min from −60° C. (−76° F.) to 315° C. (600° F.); samples comprised 7 mg of resin matrix composition (approximately 20 mg of prepreg). The Tg value measured is the “midpoint” temperature. Unless otherwise stated, DSC measurements may be carried out using this method.

TABLE 1 Tg for each batch T0 24 h @ 50° C. 72 h @ 50° C. 144 h @50° C. 152 h@50° C. 200 h@50° C. BATCH 1 −0.99° C.  1.1° C. 7.39° C. 17.63° C. 18.19° C. 29.74° C. BATCH 2   −2° C. 0.97° C. 6.24° C.  16.6° C. 19.33° C. 26.13° C. BATCH 3 −0.84° C. 2.91° C. 9.19° C. 15.41° C. 23.04° C. 27.39° C. BATCH 1* −0.24° C. 2.63° C. 8.15° C. 17.41° C. 19.71° C. 27.91° C.

Storage Simulation

The samples were subjected to a process to simulate storage of resin matrix composition chips in a bulk quantity in a storage bag under harsh conditions of storage. This simulation increased the chance that clustering or agglomeration of chips might occur A temperature of 60° C. to mimic bright sunlight and a pressure of 1 bar (14.5 psi) was applied, corresponding to the higher pressure that a chip could perceive at the bottom of a storage bag. The storage conditions were performed by using a “vacuum bag” method as set out below and with reference to FIG. 1.

The apparatus for the vacuum bag method is shown in FIG. 1 which shows a tool 1 upon which is mounted a vacuum bag 2 having a first side 2 a and a second side 2 b sealed with mastic 2 c and a sandwich arrangement comprising two layers of Teflon coated glass, referred to as “breathers”, 3 a, 3 b, layers of resin matrix composition product 4 a and 4 b and a Teflon coated glass primer 5 (length 50 mm) with which to commence the T Peel test.

Two layers of the sample resin matrix composition are mounted in the apparatus and the assembly was placed in an oven for 15 hours at 60° C. and a pressure of 1 bar was applied using a vacuum pump.

Samples Preparation and T Peel Test (in Accordance with ASTM D1876)

Once the samples had been subjected to the Storage Simulation the samples were prepared for use in T Peel tests. The T Peel test allows the adhesion strength between two layers of a sample bonded to each other to be determined using a tensile machine. An average strength of debonding between layers is measured and a “Peel torque” is determined. The “Peel torque” is equal to the average strength normalized to a 10 mm width. For each batch and each treatment, 3 samples were tested according to T Peel test ISO11339. The samples were cut to the following dimensions:

-   -   Width: 30 mm     -   Length: 300 mm     -   Depth: 2 plies     -   Orientation: 0°

The two layers of the sample were gripped respectively in a fixed jaw and a mobile jaw as shown in FIG. 2. The two layers of the sample are then separated by moving the mobile jaw away from the fixed jaw in the direction shown. The force applied in the test to separate the layers and its variation is measured using software in accordance with ASTM D1876, from which an average value of debonding strength is calculated. The Peel torque value is determined by the climbing drum peel test to determine the peel resistance of adhesive bonds in accordance with ASTM D1781. This test consists of peeling a thin strip of metal from a thick strip of resin matrix material by winding the thin strip around a drum. Torque is applied to the drum by pulling down on straps wrapped around the drum. The thin strip of metal is wound on the drum at a smaller radius than the straps. The difference in radius (i.e. moment arm) results in a large torque being applied to the drum compared with that applied on the thin strip. The resultant upward motion causes the thin strip to peel from the thicker strip. The average peel torque T can be calculated as follows:

T=(Ro−Ri)×(Fp−Fo)/b

where Ro is the flange radius, Ri is the drum radius, Fp is the average load required to bend and peel adherend (including load required to overcome the torque resistance of the drum), Fo is the load required to overcome the torque resistance of the drum and b is the specimen width. Both Ro and Ri account for one half the loading strap thickness. The upward motion of the drum causes the thin strip to be peeled from the thicker resin matrix material resulting in bond failure.

The results of the T Peel tests are set out in Tables 2 to 5. For each batch three samples were tested under each regime and the results are the mean value of the three tests.

TABLE 2 Average deboning Average Peel BATCH 1 Strength ²(N) torque ³(N/10 mm) T0 34.8 11.6 24 h @ 50° C. 38.7 12.9 72 h @ 50° C. 2.2 0.7 144 h @50° C. 0.1 0.0 152 h@50° C. 0.4 0.2 200 h@50° C. 0.2 0.1

TABLE 3 Average deboning Average Peel BATCH 2 Strength ⁴(N) torque ⁵(N/10 mm) T0 34 11.3 24 h @ 50° C. 43.2 14.4 72 h @ 50° C. 2.1 0.7 144 h @50° C. 0.4 0.1 152 h@50° C. 0.2 0.1 200 h@50° C. 0 0

TABLE 4 Average deboning Average Peel BATCH 3 Strength ⁶(N) torque ⁷(N/10 mm) T0 40.2 13.4 24 h @ 50° C. 47.1 15.7 72 h @ 50° C. 1.5 0.5 144 h @50° C. 0.9 0.3 152 h@50° C. 0.2 0.1 200 h@50° C. 0.3 0.1

TABLE 5 Average deboning Average Peel BATCH 4 Strength ⁸(N) torque ⁹(N/10 mm) T0 42.0 14.0 24 h @ 50° C. 41.4 13.8 72 h @ 50° C. 15.5 5.1 144 h @50° C. 1.7 0.6 152 h@50° C. 1.6 0.5 200 h@50° C. 1.4 0.5

A plot of the T Peel value versus Tg midpoint value for each batch was plotted and is shown in FIG. 3. ²Mean value calculated from 3 specimens results³Mean value calculated from 3 specimens results⁴Mean value calculated from 3 specimens results⁵Mean value calculated from 3 specimens results

For batches 1, 2 and 3 adhesion decreases significantly around 5° C. of Tg and debonding occurs at approximately 15° C. A slight difference is noticed with results of batch 1* in that the adhesion decreases less abruptly than with the other batches and debonding occurs at around a Tg of 20° C. The batch 1* plies were thicker than the batches 1, 2 and 3 plies which may contribute to the tailing effect observed for batch 1*. ⁶Mean value calculated from 3 specimens results⁷Mean value calculated from 3 specimens results⁸Mean value calculated from 3 specimens results⁹Mean value calculated from 3 specimens results

For every batch debonding is reduced where the Tg is 5° C. and debonding occurs completely where the Tg value is above 20° C. for chips which have been treated according to the method of the invention and subjected to a simulation of harsh storage conditions.

In summary, a resin matrix composition material which is staged so that it has a Tg over 20° C. is suitable for storage over an extended period at ambient temperature without exhibiting clustering or agglomeration irrespective of whether the storage temperature is regulated. 

1. A method of producing a stabilised uncured resin matrix composition comprising providing an uncured resin matrix composition having an initial glass transition temperature (Tg) and subjecting the resin matrix composition to: i. a first regime comprising imparting energy to the uncured resin matrix composition to raise the Tg by at least 5% to provide a raised Tg whereby the residual cure of the resin matrix is reduced by a maximum of 20%, preferably a maximum of 10% and more preferably a maximum of 5%; and ii. a second regime comprising storing the uncured resin matrix composition to provide a stabilised uncured resin matrix composition wherein the Tg of the stabilised uncured resin matrix composition is such that it does not increase by more than 10% from the raised Tg when stored for 14 days.
 2. A method according to claim 1 wherein the uncured resin matrix composition is initially subjected to a method of determining the treatment regime for stabilising the Tg of the uncured resin matrix composition comprising: i) providing an uncured resin matrix composition having an initial Tg and selecting a first regime comprising an elevated temperature to impart energy to the uncured resin matrix composition whereby the residual cure of the resin matrix is reduced by a maximum of 20%, preferably a maximum of 10% and more preferably a maximum of 5%; ii) imparting energy to the resin matrix composition in the first regime so as to increase the Tg of the resin matrix composition; iii) selecting a second regime comprising storing the resin matrix composition wherein the second regime is different to the first regime; iv) subjecting the resin matrix composition to the second regime whereby the residual cure of the resin matrix is reduced by a maximum of 10% and more preferably a maximum of 5% the resin matrix composition; v) measuring the raised Tg of the resin matrix composition prior to or at commencement of the second regime and measuring the Tg of the resin matrix composition periodically until over 14 days either: i. the periodically measured Tg does not increase by more than 10% from the raised Tg thereby determining the conditions of the first regime and the second regime; or ii. where the periodically measured Tg increases by more than 10% from the raised Tg, modifying the first regime and/or second regime and repeating steps ii), iv) and v) until the periodically measured Tg does not increase by more than 10% from the raised Tg.
 3. A method according to claim 1 wherein the resin matrix composition comprises polymer chains and imparting energy to the uncured resin matrix composition provides a reduction in cure enthalpy to complete cure which is in the range of from 2% to 20%.
 4. A method according to claim 1 wherein the raised Tg is increased by at least 5% from the initial Tg.
 5. A method according to claim 1 wherein the raised Tg is increased by at least 25% from the initial Tg.
 6. A method according to claim 1 wherein the temperature in the second regime is ambient.
 7. A method according to claim 1 wherein energy is imparted to the uncured resin matrix composition in the first regime by heating the uncured resin matrix composition to an elevated temperature for a first period of time and subjecting the resin matrix composition to pressure.
 8. A method according to claim 1 wherein energy is imparted to the uncured resin matrix composition in the first regime for a period of 1 to 20 hours.
 9. A method according to claim 1 wherein the second regime comprises storing the resin matrix composition for at least 12 hours at ambient temperature.
 10. A method according to claim 1 wherein the second regime comprises storing the resin matrix composition for a period of at least 24 hours to 10 weeks at ambient temperature.
 11. A method according to claim 1 wherein the uncured resin matrix composition is impregnated in a fibrous reinforcement material.
 12. A method according to claim 14 wherein the fibrous reinforcement material pre-impregnated with an uncured resin matrix composition comprises an unused or uncured composite.
 13. A method according to claim 1 wherein the uncured resin matrix composition is in the form of discrete elements.
 14. A method according to claim 1 wherein the stabilised uncured resin matrix composition comprises discrete elements having a tack in the range of from 0.1 to 0.6 as measured using the method as disclosed in the description of this application.
 15. A method of determining a treatment regime for stabilising the glass transition temperature (Tg) of a curable resin matrix composition comprising: i. providing a resin matrix composition wherein the resin matrix composition may be uncured having an initial Tg and selecting a first regime comprising an elevated temperature to impart energy to the uncured resin matrix composition whereby the residual cure of the resin matrix is reduced by a maximum of 20%, preferably a maximum of 10% and more preferably a maximum of 5%; ii. imparting energy to the resin matrix composition in the first regime so as to increase the Tg of the resin matrix composition; iii. selecting a second regime comprising storing the resin matrix composition wherein the second regime is different to the first regime; iv. subjecting the resin matrix composition to the second regime whereby the residual cure of the resin matrix is reduced by a maximum of 10%; vi) measuring the Tg of the resin matrix composition periodically until over a selected period of time ranging from 12 hours to 144 hours or periods thereof either: a. the periodically measured Tg does not increase by more than 1%, or 2%, or 4%, or 5% or 10% from the raised Tg thereby determining the conditions of the first regime and the second regime; or b. where the periodically measured Tg increases by more than 1%, or 2%, or 4%/o, or 5% or 10% from the raised Tg, modifying the first regime and/or second regime and repeating steps ii), iv) and v) until the periodically measured Tg does not increase by more than 1%, or 2%, or 4%, or 5% or 10% from the raised Tg.
 16. A treated uncured or unused resin matrix composition having a stable glass transition temperature (Tg) at ambient temperature wherein the uncured resin matrix composition has a raised Tg after being treated and the Tg of the treated uncured resin matrix composition remains within 10% of the initial raised Tg when stored for at least 1 day.
 17. A treated uncured resin matrix composition according to claim 16 wherein the Tg of the treated uncured resin matrix composition remains within 10% of the raised Tg when stored for at least 1 week.
 18. A treated uncured resin matrix composition according to claim 16 wherein the Tg of the treated uncured resin matrix composition remains within 10% of the raised Tg when stored for at least 1 month.
 19. A treated uncured resin matrix composition according to claim 16 in the form of discrete elements.
 20. A treated uncured resin matrix composition according to claim 19 wherein the discrete elements have a tack F/Fref of not more than 0.1 to 0.45 at ambient temperature where Fref=28.19N and F is the maximum debonding force.
 21. (canceled)
 22. (canceled) 